1. Field of the Invention
The present invention pertains to an aerodynamic drag reduction arrangement for use with high speed rotating elements and functions to reduce momentum transfers from a boundary layer established by a rotating element to a boundary layer established by a fixed element.
2. Related Background Technology and Art
Any mechanical device having a relative smooth dement that rotates in a fluid medium at a high speed generates a centrifugal flow of ambient fluid radially outwardly from the axis of rotation of the element due to the friction between the ambient fluid and the rotating mechanism, such flow being conventionally referred to as "windage." An element rotating at high speed in a dense fluid can experience a high mechanical power loss from windage drag. Generally speaking, the power input necessary to rotate a disc, for example, is proportional to the rpm of the rotating disc to the third power, the density of the fluid, the disc diameter to the fifth power, a drag coefficient which depends upon the specific geometry of the disc and the disc's Reynolds Number. The viscosity of the fluid will determine whether the flow pattern is laminar or turbulent, and will affect the thickness of the boundary layer associated with the rotating element. However, it should be noted that doubling viscosity will not double the necessary power input.
In the case of a rotating disc, the primary source of drag is from axial fluid flow entering the rotating boundary layer. As soon as the fluid enters the rotating boundary layer, the fluid is pumped radially by centrifugal forces. The result is a large axial mass flow which is accompanied by a significant momentum transfer and drag. Associated with the drag and momentum transfer, there is also heat transfer. Therefore, a reduction in drag on a rotating element will result in a reduction of heat transfer as well.
Typical environments in which the drag reduction arrangement of the present invention can be utilized to significantly reduce power consumption is electric motors and scroll fluid devices. FIG. 1 shows a typical construction of a known electric motor which includes a rotating shaft 1 that is rigidly connected to a rotating motor rotor 2. A fixed stator 3 surrounds rotor 2 and is typically formed with a plurality of circumferentially spaced slots 4 adapted to accept motor windings (not shown). Rotating shaft 1 can be journaled in a bearing 5 as is well known in the art. The radial distance typically provided between rotor 2 and stator 3 permits the axial flow and radial pumping discussed above, thereby reducing the efficiency of the electric motor by increasing the power consumption thereof.
It is well known in the art of rotating scroll fluid devices that the inertial forces of an orbiting scroll must be balanced by a rotating counterweight. As shown in FIG. 3 herein, a typical counterweight 10 is rigidly secured to a rotating shaft and comprises a dense, non-symmetric rotating mass. If used in a high speed scroll fluid device, rotating counterweight 10 can also develop axial mass flow and resulting radial pumping so as to develop significant momentum transfer and drag on the scroll fluid device.
Finally, by way of an additional example, the drag reduction arrangement of the present invention can also be used in a co-rotating scroll fluid device using opposed, meshed, axially extending, co-rotating involute wraps that experience relative orbital movement without rotating relative to each other. When such a scroll fluid device is operated at increasingly higher speeds and efficiency of the device becomes a paramount consideration, aerodynamic drag resulting from the centrifugal flow of ambient fluid generated during operation of the device must be reduced in order to minimize power consumption. If such a scroll fluid device operates as compressor, an inlet port area is provided at an outer radial zone between the meshed involute wraps. In order to minimize power consumption, it is desirable that the fluid entering the inlet port area should have as little angular momentum induced by the rotation of the co-rotating involute wraps as possible.
Therefore, them exists a need in the art for a drag reduction arrangement for use in a mechanical device including a high speed rotating element in order to minimize power consumption of the device and increase operational efficiency.